Project Summary Caries is one of the most prevalent oral diseases. Despite several decades of substantial caries research, only incremental progress has been made in disease prevention. Caries is the result of polymicrobial dysbiosis and not curable since the tooth structure is destroyed by fermentative acid by-products from caries-associated bacterial species. Therefore, strong emphasis should be on caries prevention. Caries risk assessment (CRA) is a widely accepted approach to caries prevention, but has been criticized because it leads to patients with increased risk not being identified. One drawback of CRA is that the polymicrobial nature of caries is not considered. Furthermore, it lacks any molecular markers sampled directly at the tooth surface where caries occurs. A key to improved CRA and prevention is a better understanding of molecular mechanisms that promote oral commensals associated with general oral health. We have identified an important trait of commensal oral streptococci that promotes molecular commensalism: the pyruvate oxidase, SpxB, provides a growth advantage under aerobic conditions since its metabolic activity generates additional ATP as well as pathogen-inhibiting H2O2. SpxB therefore enables early commensal streptococci to outgrow and inhibit H2O2 susceptible and caries- associated species. Our hypothesis is that commensal streptococci producing higher amounts of metabolically important ATP and inhibitory H2O2 are more competitive and thus allow dental plaque to be more resistant to cariogenic dysbiosis. We therefore propose the following specific aims Aim 1: Cellular context - molecular mechanisms determining differential H2O2 production. Aim 2: Oral context - determine whether spxB regulation influences competitiveness in vivo. Aim 3: Clinical context - Real time measurements of H2O2 production and spxB expression from ex vivo dental plaques. The proposed aims will support our overall goal to better understand the health-associated traits of oral commensals and lead to a paradigm shift in caries research towards that of molecular commensalism; a vastly understudied area of oral microbiology. The gained knowledge will provide important new insight into the development of new strategies to prevent oral diseases like caries.